The present invention relates to tubing injectors for running tubing into a wellbore, and more particularly, to a tubing injector that provides a substantially constant pressure-clamping device engaging the tubing and in which the beam is preferably segmented.
After a well has been completed to produce oil and gas, it is necessary to periodically service the well. There are many occasions when the service procedure is carried out using coiled tubing. Such coiled tubing is inserted into the wellhead through a lubricator assembly or stuffing box. Typically, this is necessary because there is a pressure differential at the surface of the well and the atmosphere, which may have been naturally or artificially created, that serves to produce oil or gas or a mixture thereof from the pressurized well. The coiled tubing is inserted by an injector which generally incorporates a tubing guide, or gooseneck, and a plurality of gripper blocks for engaging the coiled tubing and moving it through the injector. One such injector apparatus is shown in U.S. Pat. No. 5,553,668 assigned to the assignee of the present invention.
The coiled tubing is relatively flexible and can therefore be cyclically coiled onto or off of a spool, or reel, by the injector, which often acts in concert with a windlass and a power supply, which drives the spool or reel. In the injector, the gripper blocks are attached to movable gripper chains. The gripper chains are movably mounted on linear beams, which provide support for the gripper blocks and gripper chains. The gripper blocks sequentially grip the coiled tubing that is positioned therebetween. When the gripper chains are in motion, each chain has a gripper block that is coming in contact with the coiled tubing as another gripper block on the same gripper chain is breaking contact with the coiled tubing. This continues in an endless fashion as the gripper chains are driven to force the coiled tubing into or out of the wellbore, depending on the direction in which the drive sprockets are rotated.
In the past, such coiled tubing has had a constant cross section. However, maintaining a constant diameter for the coiled tubing can present problems under certain circumstances. For example, it may be desirable to reduce the weight of the string, and this cannot be done if the string has a constant diameter. Further, a larger string causes more drag in the wellbore, particularly when the strings are being used in a horizontal or other deviated position of the well. To address this problem, a tubing string has been developed which has a tapered connection between two lengths of tubing having different diameters. This is accomplished by utilizing a tapered connector which provides an elongated tapering tubing string portion which is connected at opposite ends thereof to a larger tubing portion and a smaller tubing portion. This is shown in U.S. Pat. No. 6,367,557, also assigned to the assignee of the present invention.
As coiled tubing is used in deeper and higher pressure wells, there is a need to have a coiled tubing string that is optimized for weight and strength, and the coiled tubing strings with a variable outside diameter described above are now used. Handling the transition from smaller to larger coiled tubing with the tapered connector with current surface and wellhead equipment has created operational and safety challenges.
The beams in the injectors are pressure actuated by hydraulic cylinders. When a larger diameter portion of the coiled tubing is moved into the injector, the beams will be forced outwardly which applies force to the hydraulic cylinders. Because the hydraulic fluid in the hydraulic cylinders is virtually incompressible, the force on the hydraulic cylinders increases the pressure therein dramatically. This high pressure situation can either generate more force on the coiled tubing, causing it to be damaged, or subject the hydraulic cylinders to excessive pressure or both. Any of these results is obviously undesirable. Therefore, there is a need to control the pressure used to actuate the beams. The present invention solves this problem by providing a pressure control for the hydraulic cylinders, which actuates the beams so that the pressure stays the same for different sized coiled tubing portions.
Another problem with existing injectors is that they use flat one-piece beams, which are normally parallel. As a different outside diameter portion of coiled tubing enters the injector, the beams will start to open or close so that they are no longer parallel with each other. The result is that it is difficult, if even possible, to keep the chain system in contact with the coiled tubing. The present invention solves this problem by providing a segmented beam, which allows the gripper blocks to stay in contact with the coiled tubing even when a tapered connector is moving through the injector. Each segment or section of the beams is free to move, thus making up the difference in spacing caused by the tapered connector while also maintaining a substantially constant force on the coiled tubing and maintaining a maximum effective gripper length thereon.